Histology: Tissues and Organization

• Histology is the study of microscopic structures of cells, tissues, and organs.
• Histo means tissue, Logia means branch of learning

Uses of Histology

• Definite tissue diagnosis
• Tissue culture
• Stem cell research

Organization of the Human Body

• Cells combine to form tissues.
• Tissues combine to form organs.
• Organs combine to form organ systems.
• Organ systems make up the human body.

Four Types of Tissues

• Connective tissue
• Epithelial tissue
• Muscle tissue
• Nervous tissue

Preparing and Examining Tissues and Organs

• Microtechniques are used.
• Microscopes are used.

Epithelial Tissue

• Epithelium is composed of crowded cells with very little intercellular substance.
• Cells are attached together by cell junctions
• Epithelium forms membranes that cover or line body surfaces and cavities.
• Invaginated cells from epithelial membranes to the underlying connective tissue forms the parenchyma of the glands.
• Epithelium rests on a basement membrane that separates it from the underlying connective tissue.
• Epithelium is avascular, meaning that it's not penetrated by blood vessels.
• Epithelium is penetrated by nerves.
• Epithelial tissue arises embryologically from ectoderm, mesoderm, or endoderm.

Classification of Epithelial Tissue

• According to the number of layers, epithelium can be simple or stratified.
• Simple epithelium consists of one layer of cells.
• Stratified epithelium consists of more than two layers, with the basal layer being columnar in all types and the superficial layer differing.

Simple Epithelium Types

Simple Squamous Epithelium

• Consists of a single row of flat cells with a central, flattened nucleus, resting on a basement membrane.
• It is located in lung alveoli, which allow gas exchange.
• It is also located in the inner lining of blood vessels, called endothelium.
• Simple squamous epithelium is also found in the peritoneum and pleura, called mesothelium.
• Simple squamous epithelium facilitates gaseous exchange.
• Simple squamous epithelium facilitates movement of viscera.

Simple Cuboidal Epithelium

• Composed of a single row of cubical cells with a central, rounded nucleus, resting on a basement membrane.
• It is found in thyroid follicles and tubules of the kidney.
• Simple cuboidal epithelium functions in secretion and excretion.

Simple Columnar Epithelium

• Consists of a single row of columnar cells with a basal, oval nucleus, resting on a basement membrane.
• It is located in the stomach and intestine.
• Simple columnar epithelium functions in secretion and absorption.

Simple Columnar Ciliated Epithelium

• Consists of a single row of columnar ciliated cells with a basal oval nucleus, resting on a basement membrane.
• It is located in the uterus and fallopian tube.
• Simple columnar ciliated epithelium functions in the transportation and movement of the ovum.

Pseudostratified Columnar Ciliated Epithelium

• Consists of a single row of cells resting on a basement membrane.
• Some cells are tall and reach the surface, while others are short.
• Nuclei are located at different levels.
• It is located in the respiratory system, including the trachea and bronchi.
• This type of epithelium functions in lubrication, protection, and transportation.
• Pseudo means false.

Stratified Epithelium Types

Stratified Squamous Epithelium

• Consists of many rows of cells, with the surface cells being squamous.
• The basal row is columnar, followed by many rows of polyhedral cells.
• There are two types: non-keratinized, located in the Oesophagus, and keratinized, located in the skin.
• Stratified squamous epithelium functions in protection.

Transitional Epithelium

• Consists of many rows of cells, with the surface cells being cuboidal with a convex outer surface (dome-shaped).
• The basal row is columnar, followed by many rows of polyhedral cells.
• It is located in the urinary system, including the urinary bladder and ureter.
• Transitional epithelium functions in protection and is distensible.

General Functions of Epithelium

• Protection
• Secretion
• Absorption
• Excretion
• Gaseous exchange

Glandular Epithelium Classification

• Epithelial cells proliferate and undergo downgrowth into subjacent connective tissue.
• Glands may be exocrine or endocrine.

Exocrine Glands

• They have ducts.
• They release secretions into the surface through a duct.
• Located in salivary glands.

Endocrine Glands

• They do not have ducts.
• Endocrine glands release hormones.
• Hormones are released into the bloodstream.
• Located in the pituitary and thyroid glands.

Classification of Exocrine Glands

According to the Number of Cells

• Unicellular: the only example is the goblet cell, which secretes mucus.
• Multicellular exist.

According to the Shape of the Gland

• Shape of secretory unit.
  • Tubular, alveolar shapes are possible.
• Shape of duct.
  • Non-branched duct (simple gland).
  • Branched duct (compound gland).

According to the Mode of Secretion

• Merocrine release secretion by simple exocytosis.
• Apocrine: the apex of the cell is lost with secretion.
• Holocrine: the whole cell is destroyed with secretion.

According to the Way Secretory Products Leave the Cell

• Merocrine glands: secretory granules are shed by exocytosis (e.g., salivary glands).
• Apocrine glands: the secretory product is shed together with parts of the apical cytoplasm (e.g., lactating mammary gland).
• Holocrine glands: the product of secretion is shed with the whole cell (e.g., sebaceous glands).

Specializations of Cell Surface

• Apical surface:
  • Cilia
  • Microvilli
• Lateral surface:
  • Tight junction
  • Adherent junction
  • Gap junction
• Basal surface:
  • Hemidesmosome

Apical Surface Specializations

Cilia

• Motile hair-like processes extend from the free surface of cells.
• Cilia are present in the mucus membranes of the respiratory system and the female genital system.
• Cilium is long (5-15 µm in length).
• Cilia move mucus and other substances over the surface by rapid beating in one direction.
• In the respiratory system, cilia move mucus and debris.
• In the fallopian tube, cilia move ovum towards the uterus.

Microvilli

• Thin cylindrical membrane-bound cytoplasmic processes project from the apical surface of the cell.
• The length is 1-2 µm.
• Microvillus is covered with a cell membrane and glycocalyx, and its core contains a bundle of actin microfilaments that support the microvillus and maintain its shape.
• Increases the surface area of the cell membrane.
• Found in absorptive cells of the small intestine.
• Found in cells of proximal renal tubules
• Microvilli and its cover of glycocalyx is called Brush border.
• They increase the absorptive efficiency of the cells.

Stereocilia

• Long, non-motile microvilli.
• Located in the epididymis and the inner ear.
• Increases the surface area of epithelium in the Epididymis.
• Function in the generation of hearing signals via sensory cells of inner ear.

Celiac Disease

• Caused by an immune reaction against the wheat protein gluten.
• Results in intestinal inflammation and decreased microvilli of absorptive cells, leading to mal-absorption and severe anemia.

Junctional Complex

• This is the arrangement of junctions in the epithelial cells near the apical surface.
• Includes tight junction, adherent junction, desmosome, and gap junction.

Intercellular Junctions

• Sites of special contact between cells.

Tight Junction

• Present between epithelial cells.
• Forms a seal that prevents the passage of substances through the intercellular space.
• The cell membranes of 2 adjacent cells are fused together along a system of surface ridges formed from transmembrane proteins.
• Tight junctions can be zonula occludens, which encircle the cell like a belt and are found in the endotheial cells of brain capillaries.
• Tight junctions can be fascia occludens, which are band-shaped and found in blood capillaries except the brain.

Desmosome

• Circular spot-like adherens junction.
• Found in epithelial cells and cardiac muscle.
• Located deep to the zonula adherens in epithelial cells.
• The intercellular space is occupied by the protein cadherin.
• Circular plaques of protein are attached to the cytoplasmic side of the cell membrane.
• Keratin intermediate filaments are attached to the plaques.
• Links the cytoskeleton of the 2 cells together and prevent cell separation during contractile activity.

Hemidesmosome

• It's half a desmosome.
• Present at the site of contact between the epithelial cell and the basement membrane.
• Attaches epithelial cells to the basement membrane.
• Its structure is half the structure of a desmosome.

Gap Junction

• The cell membrane of 2 adjacent cells is separated by a narrow intercellular space traversed by multiple channels that communicate the interior of the connected cells.
• Allows movement of ions and signaling molecules, and depolarization waves between the cells.
• Present in epithelial cells, cardiac and smooth muscles, bone cells, and nerve cells.

Clinical Applications

• Metaplasia is the transformation of one type of tissue to another in response to injury.
• This condition is usually reversible if the injury is removed.
• An example is the pseudostratified ciliated columnar epithelium of the respiratory passages (e.g., trachea) of heavy smokers undergoing squamous metaplasia and transforming into stratified squamous epithelium.